Conventional rotary windmills possess a plurality of windmill propeller or turbine blades for rotation either in a vertical plane about a horizontal axis or in a horizontal plane about a vertical axis. The most common type of rotary windmill is commonly referred to as a vertical wind turbine system and employs three or more propeller blades mounted on a shaft extending from a complex tower vertically mounted at a height of 400 feet or more above ground level. The vertical wind turbine system is very expensive to construct, inefficient to operate, difficult and expensive to maintain, and has caused many environmental health hazards. The propeller blades are aerodynamic shaped with a typical blade length of up to 140 feet long and a weight of up to 6.5 tons. A turbine installation of this type requires an annual inspection in which the inspectors hang from cables to perform their inspection duties. Rotation of the blades is controlled from within the complex tower using an hydraulic assembly. During operation the blades are extremely noisy, dangerous to birds and prone to failure. Moreover, the efficiency of the conventional rotary windmill system, which is dependent upon using propeller or turbine type blades, is poor since the available surface area for converting incoming air, flowing past the propeller blades, into electricity is limited in comparison to the supply of available air. This results in high inefficiency which, in combination with the high cost of maintenance, has resulted in a substantial number of rotary windmill installations of this type to be abandoned.
Alternative windmill designs have been proposed to increase the efficiency of the windmill operation and to substantially reduce maintenance costs. Many alternate windmill designs employ variations of the conventional propeller type blades. One such design is taught in U.S. Pat. No. 5,570,997 in which aerodynamically shaped blades are assembled to form foldable pairs with each pair designed to operate as a clam shell. The aerodynamically shaped blades are balanced so that the air itself causes each foldable pair of blades, facing the wind, to fold open as the windmill rotates without requiring the use of any additional motorized mechanism. The foldable pairs of blades are arranged such that during each revolution of blade rotation only one pair of blades fold open while the other pair of foldable blades remain closed. Each foldable blade has a skeleton frame formed using supporting cross-bars covered with an aerodynamic foil of very thin material constructed of a light weight metal or composite having a contoured leading and trailing edge. The leading and trailing edges of the blades control the angle of the blades relative to the hydrodynamic flow of air or wind in an attempt to maximize the surface area of the one pair of open blades in contact with the wind.
Another alternative windmill design in taught in U.S. Pat. No. 6,948,905 representing a vertical axis wind turbine system employing a plurality of wind elements arranged in pairs with each pair extending from a shaft aligned along a vertical axis and spaced vertically apart from one another. Each wind element in each pair is in the shape of a cup, bowl or half barrel having a concave surface to catch the wind flowing at the concave surface side of the cup, bowl or half barrel and to exert greater resistance to wind flow as compared to the flow of wind on the opposite convex surface side of the wind element. The wind elements function as drive units supported in a frame to form a horizontal windmill which may be affixed to a building.
U.S. Pat. No. 6,682,302 teaches an alternative windmill employing a plurality of turbine blades arranged in foldable pairs which rotate in a horizontal plane above ground level. The system is designed to increase the efficiency of converting captured air into electrical power. Each foldable pair of turbine blades forms a clam shell design with each pair of foldable blades hinged together using an hydraulic assembly such that when one foldable pair of turbine blades is forced into an open position another pair of oppositely aligned foldable turbine blades is hydraulically forced into a closed position. Each foldable pair includes an upper and lower blade constructed from a thin foil of metal and has a curved shape.